The invention relates to a novel rhodium sulfide catalyst for reduction of oxygen in industrial electrolyzers. The catalyst is highly resistant towards corrosion and poisoning by organic species, thus resulting particularly suitable for use in aqueous hydrochloric acid electrolysis, also when technical grade acid containing organic contaminants is employed.
The electrolysis of aqueous HCl solutions is a well known method for the recovery of high-value chlorine gas. Aqueous hydrochloric acid is an abundant chemical by-product, especially in chemical plants making use of chlorine as a reactant: in this case, the chlorine evolved in the anodic compartment of the electrolyzer can be recycled as a feedstock to the chemical plant. Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this context is crucially dependent on the nature and performance of the catalyst, and also on the structure of the gas diffusion electrode. Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions; the activation of gas diffusion electrodes with platinum based catalysts is well known in the art, and finds widespread application in fuel cells and electrolyzers of many kinds. However, the case of aqueous HCl electrolysis poses some serious drawbacks to the use of platinum as cathodic catalyst, as it is inevitable for the gas diffusion cathode to come at least partially in contact with the liquid electrolyte, which contains chloride ion and dissolved chlorine. First of all, platinum is susceptible to chloride ion poisoning which negatively affects its activity toward oxygen reduction; a second source of poisoning is constituted by contaminant species, especially organic species, which are in most of the cases dissolved in the by-product hydrochloric acid undergoing electrolysis. Even more importantly, the combined complexing action of hydrochloric acid and dissolved chlorine gas changes the platinum metal into a soluble salt which is dissolved away, making this material inappropriate for use in gas diffusion electrodes. Furthermore, extremely careful precautions have to be taken during the periodical shut-downs of the electrolyzers, otherwise the sudden shift in the cathodic potential, combined with the highly aggressive chemical environment, causes the dissolution of a significant amount of catalyst, and the partial deactivation of the remaining portion. While tailored procedures for planned shut-downs of the electrolyzers can be set up for additional costs, little or nothing can be done in the case of a sudden, uncontrolled shut-down due to unpredictable causes like power shortages in the electric network.
Other platinum group metals appear to follow a similar fate. For example, according to Pourbaix"" Atlas of Electrochemical Equilibria in Aqueous Solutions, finely divided rhodium metal dissolves in hot concentrated sulphuric acid, aqua regia, and oxygenated hydrochloric acid. Similarly, (hydrated) Rh2O3xc2x75H2O dissolves readily in HCl and other acids. These problems have been partially mitigated with the disclosure of the rhodium/rhodium oxide based catalyst described in concurrent U.S. patent application Ser. No. 09/013,080, filed Jan. 26, 1998, now U.S. Pat. No. 5,958,197. In particular, the rhodium/rhodium oxide system, although slightly less active than platinum towards oxygen reduction, is not poisoned by chloride ions. Also the chemical resistance to aqueous hydrochloric acid with small amounts of dissolved chlorine is sensibly enhanced with respect to platinum. However, an activation step is needed to obtain a sufficiently active and stable form, of this catalyst, and some limitations arise when such a catalyst has to be included in a gas diffusion electrode; for instance, the chemical and electronic state of the catalyst is changed upon sintering in air, a very common step in gas diffusion electrode preparations known in the art. Cumbersome and/or costly operations have to be carried out to replace this step, or to restore the active and stable form of the catalyst afterwards, as disclosed in U.S. Pat. No. 5,958,197. There is no evidence that rhodium/rhodium oxide based catalysts are more insensitive to contaminants with respect to platinum based catalysts.
It is an object of the invention to provide a novel catalyst for oxygen reduction having desirable and unexpected chemical stability towards highly corrosive media.
It is another object of the invention to provide a novel catalyst for oxygen reduction having desirable and unexpected electrocatalytic activity in presence of organic contaminants.
It is another object of the invention to provide novel gas diffusion electrodes with a novel catalyst therein having desirable and unexpected electrocatalytic properties.
It is another object of the invention to provide a novel electrolytic cell containing a gas diffusion electrode of the invention and to provide an improved method of electrolysing hydrochloric acid to chlorine.
These and other objects and advantages of the invention will become obvious from the following detailed description.
The novel electrochemical catalyst of the invention is comprised of rhodium sulfide, which may be either supported on a conductive inert carrier or unsupported. This catalyst does not require any activation step prior to its use, and surprisingly retains all of its electrocatalytic activity towards oxygen reduction in presence of chloride ions and organic molecules. Moreover, the catalyst is surprisingly not dissolved by the complexing action of aqueous hydrochloric acid/chlorine mixtures, thereby requiring no particular precautions during shut-downs when used in hydrochloric acid electrolyzers. The catalyst is preferably coated on at least one side of a web, and may be used alone, with a binder, blended with a conductive support and a binder, or supported on a conductive support and combined with a binder. The binder may be hydrophobic or hydrophilic, and the mixture can be coated on one or both sides of the web. The web can be woven or non-woven or made of carbon cloth, carbon paper, or any conductive metal mesh resistant to corrosive electrolytic solutions.
Examples of high surface area supports include graphite, various forms of carbon and other finely divided supports but carbon black is preferred.
Such catalyst coated webs can be employed as gas diffusion cathodes exhibiting cell voltages, current densities and a lifetime that could not be previously obtained under normal operating conditions, especially when used in highly aggressive environments and with low purity reactants, such as the case of electrolysis of by-product hydrochloric acid.
The catalyst may be easily prepared upon sparging hydrogen sulfide gas in an aqueous solution of a water soluble rhodium salt. Nitrogen gas may be used as a carrier for hydrogen sulfide, and a pure nitrogen flow may advantageously be used to purge excess hydrogen sulfide upon completion of the reaction. The resulting solids are recovered by filtration, washing and drying to constant weight at 125xc2x0 C., for example. The rhodium sulfide obtained in this way is unsupported (unsupported catalyst). However, when the aqueous solution of the water soluble rhodium salt further contains a suspension of a suitable conductive support, then the rhodium sulfide is preferentially deposited as nanoscopic particles on the surface of the conductive particles (supported catalyst). The resulting form of rhodium sulfide must be heated in an inert atmosphere at 550 to 650xc2x0 C., and preferably above 600xc2x0 C. to form a well defined crystalline form of rhodium sulfide catalyst. The heating may be for several hours depending on the size of the batch, and the choice of the temperature is crucial for the formation of a sufficiently stable and active catalyst.
If the temperature is too low such as 300xc2x0 C., the resulting crystallites are not well-defined and the catalyst stability is not sufficient. If the temperature is too high, i.e., 725xc2x0 C., the unsupported catalyst has excellent acid stability but does not posses adequate electrochemical activity.